1. Field of the Invention
The present invention relates to diagnostic ultrasound imaging based on a contrast echo imaging technique for acquiring a tomographic image from a reception signal including an echo reflected from a contrast medium injected into a subject to be diagnosed.
2. Discussion of the Background
Ultrasound signals have been clinically used in various fields, and one of them is an application to diagnostic ultrasound apparatus. A diagnostic ultrasound apparatus acquires an image signal through transmission and reception of an ultrasound signal toward and from a subject and is used in a variety of modes utilizing non-invasiveness of the signal. One typical type of diagnostic ultrasound apparatus produces tomographic images of a soft tissue of a living body by adopting ultrasound pulse reflection imaging. This imaging method is noninvasive and produces a tomographic image of the tissue. Compared with other medical modalities such as diagnostic X-ray imaging, X-ray CT imaging, MRI, and diagnostic nuclear medicine imaging, the imaging method has many advantages: real-time display is possible, a compact and relatively inexpensive apparatus can be constructed, patient exposure of X-rays or the like will not occur, and blood imaging is possible thanks to ultrasound Doppler imaging. The imaging method is therefore most suitable for diagnosis of the heart, abdomen, mammary gland, and urinary organs, and for diagnosis in obstetrics and gynecology. In particular, pulsation of the heart or motion of a fetus can be observed in real time through simple manipulation that in as simple as placing an ultrasound probe on a patient's surface. Moreover, since patient exposure need not be cared about, screening can be carried out many times repeatedly. Furthermore, there is an advantage that an apparatus can be moved to a bedside position for ready screening.
For screening the heart or abdominal organs, contrast echo imaging has newly been introduced and spotlighted, by which a ultrasound contrast medium is trans-venous injected into a patient for evaluating the kinetics of blood flow. Since trans-venous injection of a contrast medium is less invasive than trans-arterial injection, the method is becoming popular. The main component of the contrast medium is micro-bubbles that act as a source of reflection of ultrasound waves. The larger the amount and concentration of injected contrast medium is, the larger the effect of contrast imaging is. However, since the bubbles are crushed due to irradiation of ultrasonic waves, the time during which the effect of contrast imaging persists is shortened. Although a contrast medium characteristic of high persistency and high durability against sound pressure has been developed in recent years, the long-term persistence of the contrast medium in a human body predictably raises invasiveness.
In the contrast echo imaging, the contrast medium (i.e., substantially bubbles) is successively supplied into a region of interest set within a patient with blood flowing. It is assumed that once the bubbles existing in the region have been crushed by irradiating ultrasound waves, the effect of contrast imaging will be maintained at the next time of irradiation of ultrasound waves by bubbles newly flowing into the region. However, in effect, the bubbles will be crushed in turn before representing intensification of luminance on tomographic images, thus lowering and weakening the effect of contrast imaging instantaneously. This is because ultrasound waves are transmitted and received, normally, as many times as a few thousands per second, there is an organ parenchyma whose blood flow speed is rather slow, or there are relatively thin blood vessels whose kinetics of blood is needed.
A technique for detecting the presence or absence of blood flow in a diagnostic region by checking if luminance is intensified by a contrast medium has been adopted for the most fundamental diagnosis using a contrast medium. For more advanced diagnosis, a technique for acquiring information of a temporal change in spatial distribution of a contrast medium in a diagnostic region by detecting the spread of a change in luminance or the extent of intensification of luminance has been adopted. Also employed is a technique for obtaining a time required for an injected contrast medium to reach a region of interest (ROI) and a temporal change in luminance of echoes deriving from a contrast medium in a region of interest (i.e., time intensity curve (TIC)) or a maximum luminance.
The contrast echo imaging can also be performed effectively with harmonic echo imaging using a non-fundamental component of ultrasound waves. The harmonic echo imaging is based on separate detection of only a non-fundamental component derived due to nonlinear behaviors of ultrasound-excited bubbles in a contrast medium. Since organs in a living body tends to cause less nonlinear behaviors, a preferable contrast is given contrast echo images.
As concerning a phenomena that microbubbles are crushed by irradiation of ultrasound waves, a paper is reported, in which a fact that an imaging method referred to as flash echo imaging (or called Transient Response Imaging) intensifies luminance is reported. Theoretically, this imaging method adopts an intermittent transmission technique reduced to, for example, a rate of one frame per second, instead of the conventional condensed scanning by which a few tens of frames per second. Micro bubbles which have been fully accumulated without crushes during each intermittent interval are vanished at a time, generating a higher echo signal.
There are two ways of injecting a contrast medium at present. One is bolus injection by which a contrast medium is injected from an injector at one time, but at a slower speed. The other is sustaining injection by a contrast medium is injected little by little over a longer interval, like instillation. The bolus injection is relatively easy to inject it, provides a higher luminance level at a peak of the curve made by which a contrast medium component reaching a region of interest, and is fit for TIC measurement, but in the bolus injection, the contrast-persisting time is rather short, and the time is not stable. On the contrary, although the latter sustaining injection needs to control an injected amount using a dedicated sustaining injector, this injection has the advantage of sustaining a constant concentration of a contrast medium in a region of interest for relatively longer intervals. Hence, using the sustaining injection, even a contrast medium diluted to some extent can have the effect of contrast imaging almost equivalent to the former injection.
Therefore, from the above discussion, it can be understood that the key factors in utilizing a contrast medium in diagnosis with ultrasound waves are intensification of echoes from blood flow and a quantitative evaluation of the kinetics of blood flow.
When comparing the conventional contrast echo imaging with the above key factors, problems of the conventional contrast echo imaging become apparent. That is, longevity of microbubbles are still short due to irradiation of ultrasound waves, which is primarily resultant from physical characteristics of a contrast medium, and quantitative estimation techniques which can easily provide more detailed information about blood flow are not provided enough.
Particularly, the former problem is not simple. In the contrast echo imaging, it does not make sense that signal to noise ratios may be increased by raising transmission outputs, as done in conventional imaging. The former problem suggests that the contrast echo imaging have an optimum transmission output at a level slightly less than the transmission outputs conventionally used. Control of transmission outputs of ultrasound waves has not been carried out from such point of view.